The present invention relates generally to ophthalmic characterization, and more particularly to characterization of a retina by circular profile analysis.
Diagnostics for eye disorders typically include a detailed ophthalmic examination of the retina. For initial examination, an eye doctor will view the retina through an ophthalmoscope. For a permanent record, the retina is typically photographed with a fundus camera. A fundus photograph directly records various anatomical features of the retina, such as the optic disc, fovea, blood vessels, and lesions. The imaging capabilities of fundus photography may be enhanced by supplementary techniques. A high-contrast image of retinal blood vessels, for example, may be photographed after the injection of a fluorescent dye into the bloodstream. The resulting image is referred to as a fluorescein angiogram (FA).
More sophisticated techniques have recently been developed for diagnostics of the eye. One such technique is three-dimensional optical coherence tomography (3-D OCT). In this technique, a light beam is directed onto the retina. Part of the beam is back-reflected, and interferometric analysis of the back-reflected light yields information on the structure of the retina. By varying optical parameters of the light probe, features at different depths below the surface of the retina may be analyzed. With this process, an image of a cross-section of the retina may be generated by scanning the optical probe along a line on the retina. By rastering the optical probe across the surface of the retina, a series of cross-sectional images may be produced. The series of cross-sectional images may be used to characterize the 3-D structure of the retina, and parameters such as local retinal thickness may be measured by 3-D OCT.
Analysis of the thickness of the retina may be used to diagnose certain diseases of the eye, such as glaucoma and diabetic retinopathy. One indication of the health of the eye may be provided by comparing the retinal thickness of the patient's eye with reference data acquired from a population of healthy eyes. Progression of eye disease may also be monitored by measuring changes in retinal thickness over a period of time.
The retinal thickness is dependent on the loci (points on the retina) at which the measurements are made. The measurement loci are specified with respect to a reference point on the retina. Two common reference points are the center of the optic disc and the center of the fovea. One set of historical reference data from a large population of healthy retinas has been acquired with a Zeiss Stratus OCT 3, a commonly used instrument in the field of ophthalmology. This instrument measures the retinal thickness at loci on a circle centered at the center of the optic disc. The radius of the circle is fixed at 1.73 mm.
Thickness data is conventionally displayed on a monitor or printout as a circle divided into equally-sized arcs. For each arc, the average value of the thickness is displayed, and the arc is color coded to indicate a statistical range into which the average value falls. In a conventional display, four color codes are used. Although this simple display allows an eye doctor to rapidly assess the thickness distribution of the retina, there are several shortcomings: (a) The conventional color codes have coarse granularity. There is no mechanism to indicate whether the thickness changes abruptly or gradually from one arc to another. (b) The results are highly variable as a function of the length and orientation of the arcs. There is no mechanism for providing consistent detection of local variations. (c) There is no mechanism for detailed tracking of the growth of retinal defects over time. (d) There is no mechanism for associating anomalous thickness measurements with the presence of blood vessels or other anatomical features.
What are needed are method and apparatus for analyzing, mapping, and displaying retinal parameter measurements.